Of course the text is changed in the language file:
/language/en-GB/en-GB.com_dtregister.ini
To modify beyond just the text, you'll need to hit these files:
/components/com_dtregister/views/event/tmpl/price_header.php (price above form)
/components/com_dtregister/views/event/tmpl/confirm_price_header.php (price on confirmation page)